I have used cut down bottles before. You can use water bottels or soda or some cheap drinks. Go to the store , look at all the variety. I throw out several empty water bottles per week. I'm sure many people do. You just cut the top off. You can probbaly get differnt sizes. One time I bought something like coolaid in plastic bottles. They were like 10 cents. You could drink the stuff or pour it down the drain but you got a tube for 10 cents. These had harder plastic than the water bottles.
Anyway , some of the suggestions above sound good too.
You should also shop around for complete bags. i have seen complete ones for $20. No they are not aquaskins but they do the job and don't break the bank. Mike T used to have surf bags from as low as $20 to up to $100. Check him out at
www.capecodtackle.com .